A notification system delivers a message to a set of recipients upon detection of a pre-defined event, such as a new email message in an inbox, a new topic added to a web page, and a system outage. A notification system may also take into consideration the hours in which a recipient can be notified. For example, a user of the notification system may indicate that notification messages should only be sent to the user between the hours of 9 am to 5 pm. Accordingly, the notification system will not send notification messages to the user outside of those hours.